Hashimura Bolts

Influences of Various Factors of Bolt Influences of Various Factors of Bolt

Tightening on LooseningTightening on Loosening--Fatigue Fatigue

Failure under Transverse Vibration Failure under Transverse Vibration

Shinji HashimuraShinji Hashimura

(Kurume National College of Technology, JAPAN)(Kurume National College of Technology, JAPAN)

Paper # :2007-01-0807

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SelfSelf--Loosening and Fatigue of Bolted Joints Loosening and Fatigue of Bolted Joints

Self-Loosening and Fatigue Test

Axial

Vibration

Transverse

Vibration

Twist

Vibration

Typical Loading

Types

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Past Results of Our ResearchesPast Results of Our Researches1) If a bolt has loosened in the low cycle region, damage such as crack nucleation at

the root of the first thread of the bolt does not occur, and loosening is due to bolt rotation.

2) If a bolt has loosened in the high cycle region, fatigue cracks nucleate at the root of the first thread in the all experiments.

Loosening occurs due to loss of bolt stiffness caused by crack propagation which then leads to bolt rotation.

3) 99% of loosening-fatigue life Nf under transverse vibration is spent in order to make cracks to nucleate and propagate.

4) The loosening-fatigue life significantly depends on the transverse vibration force although the initial clamping force above a threshold does not have an influence on loosening-fatigue life.

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~ Outline ~~ Outline ~

1. Influence of bolt property classDo the transverse loosening-fatigue characteristics

depend on bolt property class?

Do the transverse loosening-fatigue characteristics improve due to being tightened to within the plastic region?

2. Influences of plastic region tightening

3. Influences of Grip Length of Bolted Joint

Influences of four different factors four different factors of bolt tightening on loosening-fatigue characteristics under transverse

vibration have been investigated.

4. Influences of Engaging Thread Length of Bolted Joint

An axial fatigue characteristic improves with an increase in the grip length. Does the transverse loosening-fatigue limit improve?

An axial fatigue characteristic improves with an increase in theengaging thread length.Does the transverse loosening-fatigue limit improve?

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Experimental ApparatusExperimental Apparatus•Frequency of transverse vibration: 50~60 Hz•Lubricant: MoS2 Grease

Fi =15 kN

Tightened by wrench

Potentiometer

Vibrated clamped part

Fixed clamped part

Test bolt

Load cell fortransverse vibration

Load cell for clamping force

Linear roller

Air vibrator

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Estimation of Nominal Stress Estimation of Nominal Stress

at the Root of the First Threadat the Root of the First Thread

gtB lPCM ⋅⋅=

3

3

32

d

lPC

Z

M gt

b

Bt

⋅

⋅⋅⋅==

πσ

330.lP

MC

gt

B =⋅

=

CC depends on the tightening condition.depends on the tightening condition.

Bending moment Bending moment MMBB due to constraining due to constraining

the engaging threads, the engaging threads,

Nominal stress at the root of the Nominal stress at the root of the

first thread first thread σσtt expressed as follows.expressed as follows.

C: coefficientC: coefficient depending on tightening depending on tightening conditions conditions

The grip length: The grip length: llgg=35mm=35mm

The engaging thread length: The engaging thread length: llee=10mm=10mm

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Table Static and Dynamic strength of bolt M10

Bolt Property ClassBolt Property Class 4.84.8 8.88.8 10.910.9

Ultimate Strength σB [MPa]

420 800 1040

Notch Factor β 2.39 3.44 3.57

Axial Fatigue Limit σw [MPa]

58 58 73

Transverse Loosening-

Fatigue Limit σtw [MPa]?? ?? ??

1. Influence of Bolt Property Class1. Influence of Bolt Property Class

•Test bolt: M10Nominal Length: la=45mmThread length: lt=27 mm

•Grip length: 35mm•Engaging length: 10mm

( )( )0

200

wT

.Twaw σσ

σσ

β

σσ

−

−⋅=

σw0:Fatigue strength of a smooth specimenσT: True ultimate strengthσ0.2:Proof stress.

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• Loosening-fatigue life obviously depends on the amplitude of transverse vibration force ∆Pt /2.

• Except for ∆Pt /2=0.25 kN, all bolts had fatigue cracks.

• There is a threshold ∆Ptw/2 between 0.25

kN and 0.35 kN corresponding to the loosening-fatigue limit.

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Fatigue Limit in Fatigue Limit in

bending bending σσσσσσσσatw atw [MPa][MPa]6363 6363 79.379.3

Loosening-fatigue limits do not almost depend on bolt property class.

920.Kaw

L

awatw

σσσ ==

Table Static and Dynamic strength of bolt M10

Property Class 4.8 8.8 10.9

Axial Fatigue Limit σaw [MPa]

58 58 73

Transverse LooseningTransverse Loosening--

Fatigue Limit Fatigue Limit σσσσσσσσtw tw [MPa][MPa]6565 68.568.5 69.869.8

The transverse fatigue limits σtw can be estimated using the axial fatigue limits σaw in a practical use

because the axial fatigue limit of bolts is known to a certain degree.

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2. Influence of2. Influence of

Plastic Region TighteningPlastic Region TighteningGenerally, the axial fatigue limit of a bolted joint improves if the bolt is tightened to within the plastic region.

Does the transverse looseningDoes the transverse loosening--fatigue limit improve?fatigue limit improve?

Test bolt: Commercial hexagon head bolt M10, Property class 4.8

Fi =15 kN

Fi =25 kN

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2. Influence of2. Influence of

Plastic Region TighteningPlastic Region Tightening

The transverse looseningThe transverse loosening--fatigue limit fatigue limit

deteriorates if the bolted joint was deteriorates if the bolted joint was

tightened to within the plastic region.tightened to within the plastic region.

This result is the opposite to the axial This result is the opposite to the axial

fatiguefatigue characteristics.characteristics.

σσσσσσσσtwtw= 65 MPa= 65 MPa

σσσσσσσσtwtw= 51 MPa= 51 MPa21% down21% down

Results of Elastic Results of Elastic

region tightening.region tightening.

Results of Plastic Results of Plastic

region tightening.region tightening.

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3. Influence of Grip Length3. Influence of Grip Length

Table Tightening conditions

Engaging length: le Grip Length: lg

20202525

3535

Generally, the axial fatigue characteristic of a bolted joint improves with an increase in the grip length.


•Test bolt: Commercial hexagon head bolt M10,Property class 10.9

•Initial Clamping Force: Fi =15 kN•Lubricant: MoS2 Grease

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The transverse looseningThe transverse loosening--fatigue fatigue

characteristics deteriorates with characteristics deteriorates with

an increase in the grip length.an increase in the grip length.

This result is the opposite to the This result is the opposite to the

axial fatigueaxial fatigue characteristics.characteristics.

∆∆∆∆∆∆∆∆PPtwtw/2= 0.36 kN/2= 0.36 kN

∆∆∆∆∆∆∆∆PPtwtw/2= 0.20 kN/2= 0.20 kN40% down40% down

llgg =25 mm=25 mm

llg g =35 mm=35 mm

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gtB lPCM ⋅⋅=

( )25=

⋅gl

gt lP

( )35=

⋅gl

gt lP

ＩＩf C does not greatly change,f C does not greatly change,

MMBB increases with an increase in increases with an increase in llgg ..

MMB B :Bending moment at Point B due :Bending moment at Point B due

to gripping threads, to gripping threads,

The apparent looseningThe apparent loosening--fatigue limit fatigue limit

decreases with an increase in decreases with an increase in llgg ..x

Grip lengthGrip length

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3. Fatigue Morphology of Broken Bolt 3. Fatigue Morphology of Broken Bolt

The section at the root of the first bolt thread is subjected to complex and varied stress due to the cyclic bending moment, cyclic shearing force and twist torque.

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4. Influence of Engaging Length4. Influence of Engaging Length

Table Tightening conditions

Engaging length: le Grip Length: lg

10103535

2020

•Test bolt: Commercial hexagon head bolt M10,Property class 10.9

•Initial Clamping Force: Fi =15 kN•Lubricant: MoS2 Grease

Generally, the axial fatigue limit of a bolted joint improves with an increase in the engaging thread length.


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The apparent transverse loosening-fatigue limit deteriorates with an increase in the engaging thread length

The apparent transverse looseningThe apparent transverse loosening--

fatigue limit deteriorates with an fatigue limit deteriorates with an

increase in the engaging thread lengthincrease in the engaging thread length

∆∆∆∆∆∆∆∆PPtwtw/2= 0.30 kN/2= 0.30 kN

∆∆∆∆∆∆∆∆PPtwtw/2= 0.20 kN/2= 0.20 kN

This result is the opposite to the This result is the opposite to the

axial fatigueaxial fatigue characteristics.characteristics.

33% down33% downlle e =10 mm=10 mm

lle e =20 mm=20 mm

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gtB lPCM ⋅⋅=

gt lP ⋅

BM

C increases with an increase in C increases with an increase in llee , ,

because the constraint of the bolt because the constraint of the bolt

thread increases.thread increases.

MMB B :Bending moment at Point B due :Bending moment at Point B due

to gripping threads, to gripping threads,

The apparent looseningThe apparent loosening--fatigue limit fatigue limit

decreases with an increase in decreases with an increase in lleebecause the apparentbecause the apparent fatigue limit fatigue limit

depends on depends on MMBB

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Improvement Effect by Several ParametersImprovement Effect by Several Parameters

In each condition, whether the fatigue limits improve or notIn each condition, whether the fatigue limits improve or not

Parameters

Property Class

~to go up~

Elastic Region

Tightening

↓Plastic Region

Tightening

Grip Length

~to go up~

Engaging Thread Length

~to go up~

Axial

Fatigue limit

Almost Same

UP UP UP

Transverse

Fatigue limit

Almost Same

DOWN DOWN DOWN

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ConclusionConclusion

1. Bolt property class has almost no influence on the loosening-fatigue characteristic of a bolted joint under transverse.

2. If a bolted joint which was tightened to within the plastic region is subjected to transverse vibration, the loosening-fatigue life and the loosening-fatigue limit under transverse vibration are reduced. This result is the contrary to the result of the axial fatigue characteristics of bolted joints.

3. The apparent loosening-fatigue limit decreases with an increase in the grip length because the bending moment at the root of the first thread is in proportion to the grip length. This result is also the contrary to the result of the axial fatigue characteristics of bolted joints.

4. If the engaging thread length is long, the apparent loosening-fatigue limit decreases although the influence of bolt body length has to be removed in order to isolate the influence of the engaging threadlength. Once again, this result is contrary to the result of the axial fatigue characteristics of bolted joints.

The main conclusions obtained in this study are summarized as follows.

Thank you very much for your attentions

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3. Fatigue Morphology of Broken Bolt 3. Fatigue Morphology of Broken Bolt

A bolt is subjected to the complicate stresses by the bending moment, shearing force and twisting torque.

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